How to do BLE characteristic.Writevalue () asynchronously ?

[BangleBeginner]

My set up is Banglejs as BLE host and HM-10 as peripheral.
The Banglejs watch has a main task to perform, and occasionally
I would like the host to write a few bytes to HM-10 via peripheral 's service/characteristics.
I follow an example in the espruino reference but struggle to understand
the concept ...... After the BLE connection is established , how to use the
"characteristic" to transmit a few bytes ?

I have included my test javascript for your comments. Thank you
for any help/pointers in advance.
score2.txt

[gfwilliams]

What you're doing looks kind of ok, it's just that you're not waiting for the connection. characteristic.writeValue is asynchronous already - it returns a promise, which completes when the write is complete. You just need to pass that promise along and only execute the next code in a .then block. So like below (see my comments for what I added)...

var c;
g.setFont("Vector", 20);

function Init() {
// return added here
return NRF.connect("68:5E:1C:31:A7:95 public", { minInterval:7.5, maxInterval:40}).then(function(ga) {
    gatt = ga;
    //g.clear();
    g.drawString("1 Connected...", 10, 35);
    g.drawString("2 Get service...", 10, 50);
    gatt.device.on('gattserverdisconnected', function(reason) {
        g.drawString(reason, 10, 160);
        E.showAlert(reason).then(function() {
            Bangle.showLauncher();
        });
    });
  return gatt.getPrimaryService("0000ffe0-0000-1000-8000-00805f9b34fb");
}).then(function(service) {
  //g.clear();
  g.drawString("3 Got service...", 10, 65);
  return service.getCharacteristic("0000ffe1-0000-1000-8000-00805f9b34fb");
}).then(function(characteristic){
  g.drawString("4 Got chara", 10, 80);
  c=characteristic;
  
  // return characteristic.writeValue("+1"); // if doing it in this promise, use return to pass the writeValue promise on...
  // return characteristic.writeValue("+2");
}).catch(function(){console.log("Something wrong");});
}

function main() {
  g.clear();
  Init().then(function() { // .then added here
    if(NRF.getSecurityStatus().connected==true){
     g.drawString("5 Write", 10, 95);
      c.writeValue("+1").then(function() { // .then added here
        c.writeValue("+2");
      });
    }
  });
}
main();